Wideband microwave hybrid



July 23, 1963 A. A. OLINER 3,098,983

WIDEBAND MICROWAVE HYBRID Filed June 29, 1960 l 3 Sheets-Sheet 1 F :L Pf/df q 610276467704) AL 56m I /0 IN V EN TOR. Air/m 4 00/1/02 y 3, 1963 A. A. OLINER 3,093,933

WIDEBAND MICROWAVE HYBRID Filed June 29, 1960 3 Sheets-Sheet 2 EVE/V M005 W l I 31; 3 I mu/M JCAE/ S 4Z n FM; A f/6H7 55970 INVENTOR.

42mm 4. 0mm? 3,098,983 Patented July 23, 1963 3,098,983 WIDEBAND MICROWAVEHYB Arthur A. Oliner, Brooklyn, N.Y., assignor to lvlerrnnac Research and Development, Inc, Flushing, N.Y., a corporation of New York Filed June 29, 196i), Ser. No. 39,671 13 Claims. ((31. 333-) The present invention relates to microwave hybrids and particularly wideband microwave hybrids provided by the present invention wherein the device may be operated over a substantially wider range of frequencies than has heretofore been possible.

The well known short slot hybrid, 3 db directional coupler, in rectangular waveguides has found widespread application despite the fact that previously known forms of this device have been characterized by a somewhat restricted operating band width. A common application of this device is for connection of transmitting and receiving apparatus to a single antenna in such a fashion that both the transmitter and receiver are operatively connected to the antenna, but the receiver is substmt-i-ally isolated from the transmitter.

The conventional short slot hybrid in operation creates a 90 phase difierence between two different modes of propagation in the device at the same time maintaining an input match for both these modes. In previously known devices a substantially 90 phase difference was maintained over only a small range of frequencies. The present invention provides means for introducing bodies of diiferent dielectric constant in the device or otherwise causing the 90 phase difierence to be maintained substantially over a much greater range of frequencies, at the same time the input matches over this frequency range are also maintained. By increasing the range of frequencies through which these operating conditions are satisfied the frequency band width of the device is substantially increased.

In addition to the above advantages and features it is an object of the present invention to provide a microwave hybrid device wherein the frequency range of the apparatus is substantially increased over that of previously known devices.

It is another object of the present invention to provide a short slot hybrid device wherein the phase difference produced for the two different modes of propagation is maintained at substantially 90 over a substantial range of frequencies.

It is a still further object of the present invention to accomplish the foregoing result by introducing dielectric materials into the short-slot hybrid in areas where they :are of substantially greater effect with respect to changing the phase of one mode of propagation than with the other and where their efiect is substantially dependout upon the frequency of the propagated wave.

Other objects and advantages will be apparent from a consideration of the following explanation in conjunction with the appended drawings in which:

FIGURE 1 is -a horizontal sectional and partially schematic view of a conventional short slot hybrid apparatus constructed in accordance with the prior art;

FIGURE 2 is a plot of the phase constant versus the free space wave number for apparatus such as that shown in FIGURE 1, presented to aid in the explanation thereof;

FIGURE 3 is a horizontal, sectional view of apparatus constructed in accordance with the present invention;

FIGURE 4 is a plot of the phase constant versus the free space wave number of apparatus such as shown in FIGURE 3 presented to aid in the explanation thereof;

FIGURE 5 is a horizontal, sectional View of an alternative form of apparatus according to the present invention;

FIGURE 6 is a plot of the phase constant versus the free space wave number for apparatus of the type shown in FIGURE 5 presented to aid in the explanation thereof;

FIGURES 7, 8, 9 and 10 are further alternative forms of the present invention shown in horizontal sectional view; and

FIGURE 11 is a plot of the phase constant versus the free space wave number in the apparatus of FIGURE 10 presented to aid in the explanation thereof.

Referring now to FIGURE 1, a standard form of short slot hybrid It} is shown. The usual short slot hybrid also includes matching elements and a diminution of the guide width in the interaction region which has been omitted from FIGURE 1 for simplicity.

The slot hybrid of FIGURE 1 has an input port 11 and two output ports 12 and 13; a 4th port is designated 14. The structure is designed so that the power incident at port 11 emerges from port 12 and port 13 and substantially no power emerges from port 14. Obviously power incident at ports 12 or 13 similarly emerges from ports 11 and 14. Port 14 is described as being conjugate to port 11. Output signals from ports .12 and 13 are substantially equal in amplitude but differ in phase from each other by approximately A detailed explanation of the operation of such a slot hybrid may be found in many reference works or in issued patents such as the Patent No. 2,739,287 to H. I. Riblet issued March 20, 1956; for the purpose of explain ing the present hivention it will sufiice to explain the operation of the slot hybrid in terms of the interaction region 15 shown in FIGURE 1.

In the interaction region 15 two modes can propagate due to the double width of the guide in this region. The lowest mode in this double width region is called the even mode and the second mode is called the odd mode. As is well known, it is the interference between these two modes which produces the desired performance.

The desired performance is achieved [by maintaining [two essential requirements associated with the interaction region, first that the difference in phase shift between the even and odd modes in the interaction region is 90, and secondly both even and odd modes in the interaction region are well matched .to the incident wave from port 11.

The present invention is illustrated as applied to hybrid structures in rectangular waveguide, but hybrid structures most commonly used in rectangular waveguide may be transformed to provide equivalent devices in other types or" transmission lines such as strip transmission or coaxial transmission line. The principles of the present invention are also applicable to hybrids for other forms of transmission line to increase their operating band width.

The previously known short slot hybrid devices have characteristically had a somewhat restricted operating band width for reasons which will be understood by reference to FIGURE 2.

FIGURE 2. is a plot of the phase constant B as a funct on of the free space wave number K. For simplicity the phase constant 8 may be considered to be' a measure of phase shift of a particular mode of propagation and the free space wave number K may be considered to be a measure of frequency.

The free space wave number K may be defined by the expression where w is the frequency expressed in radians;

aoeaesa ,u is the permeability of the filling medium, s is the dielectric constant of the filling medium and (See, for example, Equation 15 on page 11 of G. L. Ragan, Microwave Transmission Circuit, vol. 9, Rad. Lab. Series, McGraw-Hill Book C0., 1948.) Although the phase velocity and the frequency are perhaps more easily understood from an intuitive standpoint, the quantities B and K are convenient analytic tools and are used in the explanation below. As an aid in gaining an intuitive understanding of the apparatus one might, however, consider the free space wave number, K, as a measure of frequency and the phase constant, 5, as a measure (inverse) of phase velocity.

From FIGURE 2-, it will be observed that the two different modes of propagation have diiferent cut-off wave numbers but that both curves are asymptotic to the fl=K straight line a. If K corresponds to the center frequency in the usable operating range, the length of the interaction region is chosen so that AB as shown in FIGURE 2 multiplied by the length of interaction region equals 90.

It will be noted that the slopes of curves 1) and c in FIGURE 2 are unequal at K so that for any frequency 9 which departs substantially from that corresponding to K the 90 phase difierence will not obtain and the performance will be seriously deteriorated. The present invention provides a structure which serves to alter the situation represented in FIGURE 2 so that a 90 phase shift between the even and odd modes can be maintained over a substantial frequency range. This is accomplished by causing the two curves representing the even mode and the odd mode to have substantially equal slopes over a considerable range of values of K FIGURE 3 shows a slot hybrid device 2 3 incorporating features of the present invention which render it operable over a substantially wider range of frequency than previ ously known devices of this type. The slot hybrid 20 of FIGURE 3 is generally similar to the conventional form of FIGURE 1 in that it is provided with an input port 21, output ports 22 and 23 and a fourth port 24. Center wall sections 26 and 2.7 divide the device into two waveguide sections. The slot of the apparatus of FIGURE 3 is filled with a dielectric slab 28. The dielectric slab 28 is formed of a material having a relatively high dielectric constant compared with that of the air or other medium which fills the remainder of the device.

The operation of the slot hybrid 29 of FIGURE 3 may be explained as follows: The electric field at the central plane of the slot hybrid 2% is maximum for the even mode of propagation and zero for the odd mode of propagation. Accordingly the presence of the dielectric slab will aiIect the odd mode negligibly but will influence the even mode strongly. Furthermore, the concentration of the field of the even mode is greater for higher frequencies and thus for higher frequencies the effective dielectric constant controlling the propagation of the even mode may be considered to be the dielectric constant of the dielectric slab.

In the limit of sufliciently high frequencies, it will be observed from Equation 1 that the effective K will be multiplied by a factor equal to \/e' where e' is the relative dielectric constant of the dielectric slab compared to that of the medium filling the remainder of the hybrid.

Thus the [3 versus K curve for the even mode will asymptotically approach the 1ine',B=K /e at high frequencies rather than the 8=K line. The modified {3 versus K curves appropriate to the presence of the dielectric slab are shown in FIGURE 4. The straight line a is the line fi=K. Line 0 representing the phase-frequency characteristic for the odd mode is substantially unchanged from that shown in FIGURE 2 by the addition of the dielectric slab. Curve 1 showing the phase versus frequency characteristic of the even mode of propagation difiers from the corresponding curve in FIGURE 2 in that it is no longer asymptotic to the {3:K line but is asymptotic to a line ,B=K'\/e' indicated as g.

Due to the fact that the effect of the dielectric slab becomes predominant at higher frequencies it will be noted that the curve 1 has a lesser slope for a given K at low frequencies, but the curve f crosses the B=K line at some K greater than K As the result, no measured between points m and l is approximately equal to A6 measured between points k and 1', these sets of points being substantially separated along the horizontal axis (which is 'a measure of frequency). Furthermore, throughout the range of values AK, the variation in A5 is not sufiiciently different from the value of A5 measured between points i and h as to significantly deteriorate the operation of the hybrid device. The loss introduced by the dielectric slab 23 will not present any substantial problem as it is relatively thin compared with the full width of the waveguide.

To design a dielectric slab to obtain a desired uniformity of differential phase shift over a predetermined frequency range, one may determine the difference between phase constants of the even and odd modes over the desired frequency range as a function of slab thickness and dielectric constant by experimental or a combination of experimental and theoretical determinations. It is thereafter necessary only to select the combined dielectric constant and material thickness which will provide the desired comprise between bandwidth and minimum pertformance within the band which is appropriate for the particular application to which the hybrid is to be put.

It should be noted that still better parallelism between the even mode and odd mode curves over a wider range of frequency may be maintained by utilizing a dielectric slab comprising several layers of material of difierent dielectric constants.

The matching problem which is present in the design of a short slot hybrid is somewhat altered by the use of a dielectric-slab such as 28 in FIGURE 3. For a slab of the type shown in FIGURE 3 the odd mode is substantially matched by the same techniques as would be utilized without its presence. However, some alterations of the matching arrangement for the even mode of propagation will be required due to the effect on this mode from the dielectric slab 28.

The necessary matching alterations may be accomplished by matching separately each junction between the interaction region and the connecting pair of waveguides; a taper or other matching structure may be utilized for this purpose. Such a'matching arrangement has the disadvantage of requiring a matching structure length in addition to the interaction region length, but this factor will not always be prohibitive and it has the advantage of permitting a design of the matching arrangement which is not dependent on the length of the interaction region. Gne may also obtain the necessary match by providing a matched structure comprising the two junctions taken t0- gether with appropriate discontinuities inserted for matching purposes.

It will be understood that the apparatus of FIGURE 3 is shown in very simple form for clarity and to simplify the explanation but in practice the guide width in th interaction region would be reduced slightly in order to prevent the propagation of the third mode in accordance with customary practice in the art. Various other features and improvements incorporated in short slot hybrids as customary practice will in general also be applicable to shunt slot hybrids incorporating the present invention.

It will be observed that the device of FIGURE 3 functions to provide increased bandwidth by providing an area in the interaction region wherein the characteristics are altered in such a way that the phase constant of one mode of propagation is affected to a greater extent than the other in a manner which is also dependent upon the frequency. The structure of FIGURE 3 relates to a specific technique in which a slab of high dielectric material is placed in the interaction region. Other techniques also are available for practice of the present invention as illustrated in FIGURE 5 for example.

In FIGURE 5 there is shown a wideband, short slot hybrid 30 according to the present invention. The hybrid has the customary input port 31, output ports 32 and 33 and fourth port 34. The interaction region of the hybrid is indicated at 35 and extends from the end of central Wall 36 to the beginning of central wall 37 approximately. In the opening 38 between the walls 36 and 37 there is located a full height septum 39.

As before, the added septum in the interaction region is thin compared with the width of the waveguide; hence the odd mode is unaffected by the added septum, but the even mode is strongly influenced. The influence on the even mode is accentuated by designing the overall structure as a two section maximally flat filter for the even mode with pass band centered about the design center frequency. This filter efiect alters the shape of the 13 versus K curve, which is shown in FIGURE 6. In FIGURE 6 it will be seen that the odd mode curve and the even mode curve n are approximately parallel throughout the interval AK, that is the vertical distance between the curves throughout this interval does not substantially depart from the vertical distance between points p and g at the abscissa K which is the design center frequency. It will :be noted that the hybrid 30 of FIGURE has a reduced width due to the indentations 41 extending along the interaction region. It will be appreciated that the full height septum form of filter is shown by way of an example, and that any filter structure of nearly zero width may be used.

The hybrid 3%) of FIGURE 5 has the advantage that both odd and even modes are automatically matched to the incident Wave without any further matching being occasioned by the addition of the septum 39. On the other hand, in the hybrid 30 of FIGURE 5 the length of the interaction region is no longer a free parameter, being part of the filter design which produced equal slopes between the two curves. It is no longer guaranteed therefore that the phase diiference between the even and odd modes will be 90 throughout the interval wherein the even mode and odd mode curves are substantially parallel (AK in FIGURE 6). It is possible, however, to incorporate an additional free parameter to guarantee the simultaneous achievement of match, 90 phase shift, and substantial parallelism between the mode curves.

One of the numerous ways in which the additional free parameter can be introduced is by the incorporation of a ridge 42 in each half of the interaction region as shown in FIGURE 5.

The ridging affects the even and odd modes difierently and causes a change in the propagation constants of the two modes, and thus serves as a firee parameter which maybe adjusted to provide the desired match, phase shift, and parallelism between mode curves.

The ridges are located at the nulls of the electric field for the third mode so that this mode is unaffected; otherwise the propagation of the third mode would be enhanced to the detriment of the operation or" the device. The spacing L is an appropriate spacing resulting in a resonant cavity for the even mode formed between the two bifurcation discontinuities. The length W corresponds to a quarter wavelength in the even mode so that the total length 2L+W of the interaction region (taking into account the various junction discontinuities present) corresponds to a maximally flat filter for the even mode.

Turning screws 43 may be provided as a final adjusting mechanism to provide the proper position and flatness of the filter characteristics.

The several parameters capable of adjustment in the device of FIGURE 5 are calculated to provide a particularly high degree of improvement in bandwidth characteristic for the microwave hybrid.

It should be appreciated that the ridges 42 are only one of enumerable structures which may be utilized to obtain an additional free parameter in the design of apparatus similar to that shown in FIGURE 5'. The possible variations will be appreciated to some extent from the description of some alternative forms of apparatus which follows. However, the number of possibilities is by no means exhausted and it will be understood that the scope of the invention shall include equivalent structures introduced to alter the propagation characteristics by introducing an additional free parameter.

FIGURES 7 and 8 show alternative forms of the invention in which the mode of operation is similar to that of the device of FIGURE 5, that is the operation is based on what may be termed the filter principle.

In FIGURE 7 a hybrid 50 is shown having an input 51, outputs 52 and 5'3, and a fourth terminal 54.

The hybrid 50 is divided into two parts by the wall sections 56 and 57 and there is a slot between the two sections at 58. A full height septum 59 is placed in the center of the slot 58. The foregoing elements of the hybrid 50 correspond generally to the construction of the device of FIGURE 5; however, the partial height ridges in the device of FIGURE 5 are replaced by dielectric slabs 62 in FIGURE 7 located in a corresponding position. The operation of the device of FIGURE 7 generally corresponds to that of FIGURE 5 already explained.

FIGURE 8 shows a slot hybrid 63 having an input port 64, output ports 65, and 66, and a fourth port 67. The hybrid is divided into two sections by dividing walls 69 and 71; in the interaction regions 68 a slot 72 joins the two portions of the hybrid. In the slot 72 there is a full height septum so that the device of FIGURE 8 corresponds to this extent to that of FIGURE 5.

The hybrid 63 does not have partial height ridges as does the device of FIGURE 5 but rather has dielectric blocks 74 located adjacent the outside walls of the hybrid in the interaction region; these blocks discriminate between even and odd modes and introduce an additional free parameter for the purpose explained in the discussion of the device of FIGURE 5.

It has previously been explained how the slope of the line representing a plot of phase constant versus free space wave number can be controlled so that the lines representing the even and odd mode can be caused to have a substantially equal slope at an abscissa corresponding to approximately the designed center frequency of the device. In an even further refinement of the invention still wider band characteristics may be achieved by controlling in addition to the slope, which is the first derivative of 5 with respect to K, the second derivative of ,8 with respect to K. This requires the introduction of still another free parameter.

When reference is made to the first (or the second) derivatives of ,8 with respect to K for the two modes being equal to each other, it will be understood that, at the same time, the first (or the second) derivatives of phase velocity with respect to frequency for the two modes will be equal to each other. The latter relation may be more meaningful from an intuitive standpoint, but in either case the result is the same in the present context.

A device utilizing second derivative control of the 18 versus K characteristic is shown in FIGURE 9. The hybrid 75 is provided having an input port 76, output ports 77 and 78 and a fourth port 79. The hybrid is divide dinto two sections by wall sections 82 and 83. In the interaction region 81 a port, or slot 84 joins the two sections.

A full height septum 85 is located in the slot 84 and partial height ridges 86 are provided in the interaction region 81 to introduce a first additional parameter which allows equalization of the slope of the versus K curves for the even and odd modes, as previously explained in connection with FIGURE 5. To this structure, which is basically that of FIGURE 5, dielectric slabs 87 are added adjacent the outer walls of the hybrid in the interaction region. This provides a second additional parameter which may be varied to control the second derivative of ,8 versus K for the even and odd modes to provide near parallelism for the two lines over a still Wider range of frequency. This refinement of the invention will be better understood in connection with the more detailed explanation of an alternative form of the invention shown in FIGURE 10.

The hybrid shown in FIGURE is a device employing distribution of the dielectric constant in the cross-section to-achieve wideband operationland thus is a refined version of the device of FIGURE 3.

In FIGURE 10 the hybrid 88 has an input port 89, output ports 90 and 91 and a fourth port 92. The hybrid is separated into two sections except in the interaction region 93; the separation is provided by center wall sections 94 and 95. The slot between wall sections 94 and 95 is filled with a dielectric slab 96 having a dielectric constant which will be designated 6 1- Slabs 97 of still higher dielectric constant are located along the outer walls of the hybrid 88 and extend along the length of the interaction region 93.

The operation of the hybrid '88 of FIGURE 10 will be better understood by reference to FIGURE 11 showing an approximate plot of phase constant versus free space wave number for the device of FIGURE 10 constructed in accordance With the present invention. The straight line :1 represents the line 9: K; as explained with reference to FIGURE 1, both the even mode curve 1' and the odd mode curve w would be asymptotic to the line a in the absence of any added dielectric materials in the hybrid 88.

Due to the fact that dielectric slabs and 7 in FIG- URE 10 are thin relative to the width of the waveguide sections, and due to their placement, they are relatively ineffective until certain higher frequencies are reached. Therefore for lower frequencies at the left of the plot in FIGURE 11 the curve 2' and the curve s are essentially asymptotic to the line a.

Flt will be noted that the dielectric slabs 97 are sufficiently that their effect does not become substantial until a still higher frequency than that frequency at which the center dielectric slab 96 becomes of substantial effect. Furthermore the outer dielectric slabs 97 affect both the even and odd modes, but to a somewhat greater extent the odd mode.

Referring back to FIGURE 11 it will be observed that at the point v on curve r the frequency has become sufficiently high so that the even mode is affected substantially by the center slab 96 and becomes approximately asymptotic to the line t representing 5=K /s' this effect is substantially the same as described with reference to FIGURE 4 at frequencies in the vicinity of point It on curve r and point w on curve s; the frequency here is not high enough so that the effects of outer dielectric slabs 97 are evident (center slab 96 will not substantially afiect the odd mode regardless of the frequency).

It was noted in the explanation of FIGURES 3 and 4 that in the absence of the outer dielectric slabs 97 the even and odd mode curves began to diverge for frequencies higher than the design center frequency of the device. In the apparatus of FIGURE 10, however, the effect of the outer dielectric slabs 97 become substantial within the design frequency range. As these dielectric slabs have a higher dielectric constant 6' than the dielectric constant of the slab 96, 5' the curves r and s which were asymptotic to the lines I and a respectively for lower frequencies both become asymptotic to the line u representing ,6 /e for higher frequencies. Furthermore the efiect of slabs 97 is greater for the odd thermore the effect of slab F7 is greater for the odd mode so that the tendency of the odd mode and even mode lines to diverge seen in FIGURE 4 is counteracted and these lines are maintained approximately parallel over a wider range of frequencies. Obviously the lines for the even and odd modes must eventually converge as they are both asymptotic to the line fl=K. /e'

The foregoing practical explanation may be converted into mathematical terms, in which case it may be stated that the center slab provides relative control of the rst derivative of the phase constant with respect to free space wave number for the two modes while the outer slabs E7 provide relative control of the second derivative in (2K2 *From the foregoing explanation it will be seen that the present invention provides a simple means for achieving a significant increase in the bandwidth of microwave hybrid devices and provides more refined techniques whereby still further increases in bandwidth of microwave hybrid devices can be achieved. In addition to the several modifications and variations of the invention shown or suggested, numerous other variations and modifications will be apparent to those of ordinary skill in the art. It is therefore desired that the scope of the invention shall not be limited to the embodiments shown or suggested but shall be limited solely by the appended claims.

What is claimed is:

1. Wideband microwave hybrid apparatus operative over a substantial frequency range centered at a predetermined midrange frequency comprising a first rectangular waveguide structure of conductive material, a second rectangular waveguide structure of conductive material having a narrow wall at least partly in common with a narrow wall of said'first waveguide structure, an aperture in the conductive material of said common wall portion providing an interaction region for said waveguide structures, said waveguide structures being capable of supporting first and second modes of propagation in said interaction region in the vicinity of said aperture, and means placed in said interaction region to afiect one of said modes to a substantially greater extent than the other for differentially altering the phase velocity for said two modes of propagation, said means comprising a structure having @a thickness in the direction of the wide transverse dimension of said waveguide structures which is small compared to the width of one of said waveguide structures, the relation between the dimensions, location, and differential phase velocity efiect of said structure with respect to said two modes causing the first derivatives of phase velocity with respect to frequency for the said two modes to be substantially equal to each other at a frequency within the design frequency range of said apparatus.

2. Apparatus as claimed in claim 1 wherein said means for difierentially altering the phase velocity for said two modes comprises a substantially flat frequency response filter structure, said filter structure being located in said aperture and having a width measured in the direction of the maximum transverse dimension of said waveguide structures which is small compared to the said maximum transverse dimension of one of said waveguide structures, and a further structure in each of said waveguide structures substantially distributed longitudinally through said interaction region in the vicinity of said aperture and located to effect one of said modes more strongly than the other and to cause a difference in the respective propa gation constants of said two modes for introducing a free parameter allowing a relative phase shift of said two modes to be maintained at approximately 90 throughout the operative frequency of said apparatus.

3. Apparatus as claimed in claim 2 wherein said fur ther structure isa ridge of conductive material extending longitudinally substantially parallel to the central axis of each of said waveguide structures.

4. Apparatus as claimed in claim 3 further including tuning means extendable into said aperture.

5. Apparatus as claimed in claim 2 wherein said further structure is a relatively thin slab of dielectric material extending longitudinally substantially parallel to the central axis of each of said waveguide structures.

6. Apparatus as claimed in claim 2 further including a thin slab of dielectric material extending longitudinally at least for the length of said aperture in each of said waveguide structures near the wall thereof opposite said aperture, said dielectric slab having a dielectric constant and dimensions for differentially altering the phase constants of said two modes of propagation at different frequencies to cause the second derivatives of the phase velocity with respect to the frequency for the said two modes to be substantially equal at a value of said trequency within the design frequency range of said apparatus.

7. Wideband microwave hybrid apparatus operative over a substantial frequency range centered at a predetermined mid-range frequency comprising a first rectangular waveguide structure of conductive material, a second rectangular waveguide structure of conductive material having a narrow wall at least partly in common with a narrow wall of said first waveguide structure, an aperture in the conductive material of said common wall portion, said waveguide structures being capable of supporting first and second modes of propagation in the vicinity of said aperture, and a bounded volume substantially distributed longitudinally through the vicinity of said aperture and having a substantial thickness in the direction of the wide transverse dimension of said waveguide structures, which thickness is small compared to the width of one of said waveguide structures, said volume having a dielectric constant which is different from the medium filling the remainder of the structure and being placed to aifect one of said modes to a substantially greater extent than the otherr 8. Wideband microwave hybrid apparatus operative over a substantial frequency range centered at a predetermined mid-range frequency comprising a first rectangular waveguide structure of conductive material, a second rectangular waveguide structure of conductive material having a narrow wall at least partly in common with a narrow wall of said first waveguide structure, an aperture in the conductive material of said common wall portion, said waveguide structures being capable of supporting first and second modes of propagation in the vicinity of said aperture, and a bounded volume in the vicinity of said aperture having a substantial thickness in the direction of the wide transverse dimension of said waveguide structures, which thickness is small compared to the width of one of said waveguide structures, said volume having a dielectric constant substantially different from the medium filling the remainder of the structures and being placed to affect one of said modes to a substantially greater extent than the other, the relation between the dimensions, relative dielectric constant and location of said volume causing the first derivative of phase constant with respect to free space wave number for the said two modes to be substantially equal at a value of said free space wave number within the design frequency range of said apparatus.

9. Wideband microwave hybrid apparatus operative over a substantial frequency range centered at a predetermined mid-range frequency comprising a first rectangular waveguide structure of conductive material, a second rectangular waveguide structure of conductive material having a narrow wall at least partly in common with a narrow wall of said first waveguide structure, an aperture in the conductive material of said common wall portion, said waveguide structures being capable of supporting first and second modes of propagation in the vicinity of said aperture, and a slab of dielectric material placed substantially distributed longitudinally through said aperture having a thickness in the direction of the wide transverse dimension of said waveguide structures which is large compared to the thickness of said common wall and small compared to the width of one of said waveguide structures, the relation between the dimensions, locations and dielectric constant of said slab causing the first deriva tive of phase constant with respect to free space wave number for the said two modes to be substantially equal at a value of said free space wave number within the design frequency range of said apparatus.

10. Apparatus as claimed in claim 9, fuather including a .thin slab of dielectric material extending longitudinally in each of said waveguide structures and placed to afiect the phase constant of one of said modes more strongly with changes in frequency than the other of said modes, said dielectric slab having a dielectric constant and dimensions for difierentially altering the phase constants of said two modes of propagation at diiferent free space wave numbers to cause the second derivative of the phase constant with respect to the free space wave number for the said two modes to be substantially equal at a value of said free space wave number within the design frequency range of said apparatus.

I l. Apparatus as claimed in claim 9 further including a thin slab of dielectric material extending longitudinally in each of said wave guide structures near the wall thereof opposite said aperture, said dielectric slab having a dielectric constant and dimensions for differentially altering the phase constants of said two modes of propagation at different free space wave numbers to cause the second derivatives of the phase constant with respect to the free space wave number for the said two modes to be substantially equal at a value of said free space wave number within the design frequency range of said apparatus.

l2. Wideband microwave hybrid apparatus operative over a substantial frequency range centered on a predetermined midrange frequency comprising a first transmission line structure, a second transmission line structure at least partially adjacent thereto, said structures having a common portion capable of supporting first and second modes of propagation, said common portion being dimensioned to cause a hybrid relation between the (terminals of said transmission line structures, and a slab of high dielectric material placed in said common portion to affect one of said modes to a substantially greater extent than the other for differentially altering the phase constants for said two modes of propagation, said slab having a thickness measured transversely to the direction of propagation in said transmission line structure which is small compared to the width of one of said wave transmission line structures, the relation between the dimensions, location, and differential phase constant effect of said sl-a b causing the first derivative of phase constant with respect to free space wave number for the said two modes to be substantially equal at a value of said free space wave number within the design frequency range of said apparatus.

13.Wideband microwave hybrid apparatus operative over a substantial frequency range centered on a predetermined mid-range frequency comprising a first transmission line structure, a second transmission line structure at least partially adjacent thereto, said structures having a common portion capable of supporting first and second modes of propagation, said common portion being dimensioned to form an interaction region for said trans- 3,098,983 I 11 12 mission linest-ruotures and to cause a hybrid relation be of said frequency within the design frequency range of tween the terminals of said transmission line structures, said apparatus. and means placed in said interaction region to affect one of said modes to a substantially greater extent than the References Cited in the fil f this patent other for difierentially altering the phase velocity for said 5 two modes of propagation, the relation between the di- UNITED STATES PATENTS mensions, location, and differential phase velocity efiect 2,739,287 Riblet Mar. .20, 1956 of the last said means causing the first derivatives of 2,876,421 Riblet Mar, 3, 1959 phase velocity with respect to frequency for the said two 2,894,216 Crowe July 7, 1959 modes to be substantially equal to each other at a value 10 2,939,092 Cook May 3'1, 1960 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,098,983 July 23, 1963 Arthur A. Oliner It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 14, before "3" insert or column 4 line 33, for "comprise" read compromise column 7,

line 74, for

[M A; 2 read B K a I 2 line 75, strike out "thermore the effect of slabs 97 is greater for the odd".

Signed and sealed this 11th day of February 1964.

(SEAL) Attest:

EDWIN L. REYNOLDS ERNEST W. SWIDER Attesting Officer Ac ting Commissioner of Patents 

1. WIDEBAND MICROWAVE HYBRID APPARATUS OPERATIVE OVER A SUBSTANTIAL FREQUENCY RANGE CENTERED AT A PREDETERMINED MIDRANGE FREQUENCY COMPRISING A FIRST RECTANGULAR WAVEGUIDE STRUCTURE OF CONDUCTIVE MATERIAL, A SECOND RECTANGULAR WAVEGUIDE STRUCTURE OF CONDUCTIVE MATERIAL HAVING A NARROW WALL AT LEAST PARTLY IN COMMON WITH A NARROW WALL OF SAID FIRST WAVEGUIDE STRUCTURE, AN APERTURE IN THE CONDUCTIVE MATERIAL OF SAID COMMON WAL PORTION PROVIDING AN INTERACTION REGION FOR SAID WAVEGUIDE STRUCTURES, AND WAVEGUIDE STRUCTURES BEING CAPABLE OF SUPPORTING FIRST AND SECOND MODES OF PROPAGATION IN SAID INTERACTION REGION IN THE VICINITY OF SAID APERTURE, AND MEANS PLACED IN SAID INTERACTION REGION TO AFFECT ONE OF SAID MODES TO A SUBSTANTIALLY GREATER EXTENT THAN THE OTHER FOR DIFFERENTIAL ALTERING THE PHASE VELOCITY FOR SAID TWO MODES OF PROPAGATION, AND MEANS COMPRISING A STRUCTURE HAVING A THICKNESS IN THE DIRECTION OF THE WIDE TRANSVERSE DIMENSION OF SAID WAVEGUIDE STRCUTURES WHICH IS SMALL COMPARED TO THE WIDTH OF ONE OF SAID WAVEGUIDE STRUCTURES, THE RELATION BETWEEN THE DIMENSIONS, LOCATION, AND DIFFERENTIAL PHASE VELOCITY EFFECT OF SAID STRUCTURE WITH RESPECT TO SAID TWO MODES CAUSING THE FIRST DERIVATIVES OF PHASE VELOCITY WITH RESPECT TO FREQUENCY FOR THE SAID TWO MODES TO BE SUBSTANTIALLY EQUAL TO EACH OTHER AT A FREQUENCY WITHIN THE DESIGN FREQUENCY RANGE OF SAID APPARATUS. 